This invention relates to sound motion picture cameras, and particularly, to sound motion picture cameras in which a film motor drives film past an exposure aperture and a capstan motor drives the film past a sound recording station.
Motion picture cameras generally form a film loop of predetermined length between a film advancing mechanism at the exposure aperture and a capstan mechanism at the sound recording station as the film strip is fed from a supply to a takeup spool. When the capstan motor at the capstan mechanism and the film motor in the film mechanism operate simultaneously in response to a release, there is a great possibility of creating an unduly large slack in the film strip between the capstan and the film advancing mechanism, usually in the form of a pull-down mechanism, which could lead to jamming of the film strip.
To avoid this, prior art cameras have devices to drive the capstan motor first so as to regulate the length of the looped film strip, and then drive the film motor only after the film strip has been shortened. This prevents jamming of the film strip. However, in this type of sound motion picture camera, because driving of the capstan motor always precedes initiation of the intermittent advancement of the film strip past the exposure aperture, even when a film cartridge is inserted into the camera with a correct predetermined loop length, the capstan motor may pull the film strip and displace the normal picture frame from alignment with the exposure aperture prior to exposure. Upon reproduction, an image flow may occur at the first picture frame.
To prevent this, attempts have been made to hold the film stationary in the normal position by increasing the force of the side pressure plates in the film advancing mechanism so that they can resist the capstan mechanism. However, this requires increasing the force of advancing the film past the exposure aperture and results in disadvantageous increase in the consumption of electrical energy by the film motor. Moreover, the side pressure plates tend to press the film laterally and the film may distort to the point where the sharpness of the images decreases. Another disadvantage arises from the fact that adjusting the operation of the side pressure plates to an ideal or design value is time consuming and troublesome. This increases production costs. As a further point, this method must permit the pinch roller and the capstan which sandwich the film to slip. Hence, the possibility of scratching the film may increase and, at the same time, the film drive by the pinch roller and capstan may be adversely affected.